feat: add 4 default income categories, localize category display names

Fresh installs (this app is published on GitHub) only got Salary,
Freelance, Investments, Gifts, Other Income by default — adds Business
Income, Rental Income, Benefits, and Item Sales, rounding out the 8-slot
categorical palette. Only affects new databases (seeding is a one-time
no-op once any category exists).

Also fixes CategoryList.tsx (added last commit) to run category names
through translateCategoryName() like every other category display in the
app, and registers the new defaults' translation keys.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/app/db/seed.py

@@ -31,6 +31,10 @@ DEFAULT_INCOME_CATEGORIES = [
     ("Freelance", "briefcase", "#1baf7a"),
     ("Investments", "trending-up", "#4a3aa7"),
     ("Gifts", "gift", "#e87ba4"),
+    ("Business Income", "building-2", "#eb6834"),
+    ("Rental Income", "key", "#eda100"),
+    ("Benefits", "hand-coins", "#008300"),
+    ("Item Sales", "tag", "#e34948"),
     ("Other Income", "plus-circle", "#898781"),
 ]